Attractions Between Molecules orIntermolecular Forces (IM Forces)

Chapter 8.4

Learning Objectives

• Understand the difference between intramolecular and intermolecular forces

• Identify the 4 common types of intermolecular forces

We have already talked about intramolecular forces.

Intermolecular forces are attractive forces between molecules.

Intramolecular forces hold atoms together in a molecule.

Covalent bonds would be an example.

Intermolecular Forces

Intramolecular = strong

Intermolecular = weak

They do control physical properties such as boiling and melting points, vapor pressure, and viscosity

Types of Intermolecular Forces

• Dipole-dipole interactions

• Ion-Dipole interactions

• London dispersion forces

Van der Waals Forces

Hydrogen bonding

Dipole-Dipole Interactions

Molecules that have permanent dipoles are attracted to each other.

Ion-Dipole InteractionsAttractive forces between an ion and a polar molecule

Ion-Dipole Interaction

The larger the charge the stronger the force

London Dispersion Forces

Occasionally electrons wind up on the same side of the atom.

London Dispersion Forces

At that instant, the helium atom is polar.

London Dispersion Forces

This polar helium atom then induces a dipole

on a neighboring helium atom.

London Dispersion Forces

London dispersion forces, or dispersion forces, are attractions between an instantaneous dipole and an induced dipole.

London Dispersion Forces

• These forces are present in all molecules, whether they are polar or nonpolar.

• The tendency of an electron cloud to distort in this way is called polarizability.

Types of Intermolecular Forces

• The dipole-dipole interactions experienced when H is bonded to N, O, or F are unusually strong.

• We call these interactions hydrogen bonds.

Hydrogen Bonding

Hydrogen Bonding: Water

Hydrogen Bonding: Water